1. Field of the Invention
This invention relates, in general, to frequency-to-binary converters and, more specifically, to decoders for two-tone communication systems.
2. Description of the Prior Art
Communication systems which are used to transmit signals along power distribution lines are susceptible to high noise conditions. Consequently, such communication systems are designed to be reliable when operating under very adverse signal-to-noise ratio conditions. When the information to be transmitted can be coded in digital form and the maximum base band information rate is several orders of magnitude below the carrier frequency of the communication system, the use of a two-step frequency modulation method greatly reduces the effects of the noise at the power line frequency and at its harmonics, thus enhancing the reliability of the communication system. A communication system using this technique is disclosed in U.S. Pat. No. 3,967,264, issued June 29, 1976 which is assigned to the assignee of this invention.
Communication systems of the type which use two-tone signals employ a decoder which processes the two-tone signals and converts the information contained therein into binary form. Some conventional decoders employ monostable multivibrator circuits which are triggered to produce a pulse signal having pulses of constant amplitude and width. Since monostable circuits are very sensitive to noise, primarily because they are triggered pulse circuits, noise pulses of short duration can be detrimental to the ability of the decoder to properly generate the desired binary signal.
Therefore, it is desirable, and it is one of the objects of this invention, to provide a two-tone decoder for converting a two-tone frequency coded signal into a binary output signal which is highly immune to noise, and which does not use a monostable multivibrator circuit which is responsive to pulses of short duration.